The purpose of this study is to determine whether D2E7 is effective and safe in the treatment of RA when taken in combination with methotrexate. Methotrexate is increasingly being used to treat RA, and it is likely that potential new treatments, such as D2E7, will be used in the future together with methotrexate. The potential for future combined use of methotrexate and D2E7 is the reason for this drug study.